This invention is directed generally to improvements in color television receivers, and particularly to an improved chroma amplifier and color killer for use in such a receiver.
Color television receivers generally include a chroma amplifier for amplifying the color components of a composite video signal. Typically, the chroma amplifier is in the form of a differential amplifier whose gain is varied by an ACC (automatic color level control) voltage for holding substantially constant the peak-to-peak amplitude of the chroma amplifier's output signal.
As the gain of the amplifier is varied by the ACC voltage, a varying D.C. voltage is developed across the amplifier's load, thereby generating a D.C. voltage, usually superimposed on the amplified chroma signal, which is indicative of the gain at which the chroma amplifier is operating. That D.C. voltage is filtered to remove the A.C. chroma signal, and the filtered D.C. signal is used to operate a color killer.
Although such chroma amplifiers perform reasonably well, their drawback is the need for a filter capacitor to remove the chroma signal from the D.C. voltage used for color killer actuation. Although the inclusion of a filter capacitor increases the cost of the amplifier, the more serious problem is that the capacitor requires an extra pin on an integrated circuit in which the chroma amplifier is frequently built.
Another problem is associated with some prior color killers. The killer voltage described above usually increases as the gain of the chroma amplifier increases. At some point near maximum gain, the killer voltage is expected to be of a predetermined value which is designed to actuate the color killer. At that point, the killer voltage may be near the circuit's supply voltage, thereby leaving little tolerance for variation in the point where color is to be killed.
Further, chroma amplifiers which develop the above-described type of killer voltage frequently need a relatively large load resistor across which the color killer voltage is developed because the color killer current has a low amplitude as the kill point is approached. The large value load resistor necessarily results in a large color killer gain. Consequently, any noise present in the chroma amplifier tends to be unduly amplified in the killer voltage.
The problems mentioned above and other problems associated with prior chroma amplifiers has rendered their performance less than perfectly satisfactory, particularly with respect to their construction in integrated circuit form.